1. Field of the Invention
The present invention is directed to an x-ray diagnostic installation of the type having a high-voltage generator which supplied an x-ray tube which emits an x-ray beam, a solid-state image converter arranged in the x-ray beam and having light sensitive pixel elements arranged in a matrix, and a control unit connected to the image converter for controlling the high-voltage generator dependent on the x-ray dose.
2. Description of the Prior Art
German Patentschrift 21 35 205 discloses an x-ray diagnostic installation having an automatic exposure unit, by means of which a criterion representative of the accumulating x-ray dose is acquired during the x-ray irradiation. So that the ongoing radiation can be shut off after a predetermined value is reached.
In x-ray diagnostic installations such as disclosed in German Patentschrift 21 35 205, which employ an x-ray image intensifier as image converter, it is known to arrange an air-filled ionization chamber in front of the x-ray image intensifier as an auxiliary component. The slight current which arises between two electrode plates having a potential difference therebetween is directly proportional to the dose rate of the incident, ionizing radiation. The dose is determined by integration. Such an ionization chamber arranged in front of the x-ray image intensifier, however, undesirably attenuates the x-radiation.
When individual x-ray pictures or image series are made without prior fluoroscopy with exposure regulation, then a dose or exposure measurement must still ensue during the x-irradiation so that the x-ray source can be shut off when a predetermined value is reached.
A "real-time" measurement during an x-ray pulse is not possible given an x-ray solid-state image converter, for example an aSi:H x-ray image detector, because it operates in a storage mode. The image information is therefore only available some time after the end of the x-ray pulse, so that a dose measurement is only possible at that time.
X-ray examination systems are known which employ a sensor matrix which each matrix member being composed of a photosensor and a storage capacitor, whereby one of the terminals is connected to a switch and the other is connected to an additional cooperating electrode. European Application 0 486 102 discloses that the current in the cooperating electrode be measured in such a matrix after brief-duration closure of the switch, this current being representative of the momentary exposure and being utilizable for an exposure measurement. A read-out of the charge of the image stored in the sensor elements ensues after the exposure, however, so that it cannot serve for exposure measurement.